On the influence of free stream turbulence on roughness-induced boundary-layer transition

This study experimentally examines how free stream turbulence (FST) alters roughness-induced transition by exposing cylindrical roughness elements of diameter-to-height ratios η =1,2,3 to three FST levels ( Tu≈ 0.05\,% , $1.15\,%$ , $1.55\,%$ ). Although the roughness Reynolds number Reₖₖ is varied solely through the free stream velocity, a detailed characterisation of the incoming FST and resulting Klebanoff modes shows that their variation remains moderate, enabling a controlled assessment of FST effects. The FST consistently promotes earlier transition for all η at subcritical global-instability Reynolds numbers, but its influence on the underlying instability mechanisms depends strongly on η . For the ‘thick’ cylinders ( η =2,3 ), the global instability remains remarkably robust: the dominant varicose mode and its frequency scaling show virtually no sensitivity to FST. In contrast, the ‘thin’ cylinder ( η =1 ) exhibits an extreme susceptibility: already moderate FST suppresses the sinuous mode predicted by global stability theory at supercritical Reynolds numbers. These findings provide a clear, experimentally grounded explanation for why global stability predictions often fail to materialise the structures in real, turbulence-contaminated environments and establish FST as a decisive factor governing whether, when and how roughness-induced instabilities manifest.